Two-space notchers are groups that consist of a chain on the second line, interrupted by a notch of two empty spaces on the second line and two to four stones on the third. The term was invented by James Davies. See also Notchers, One-Space Notchers and Three-Space Notchers.
The rule for two space notchers is, depending on how you want to formulate it, either "Five die, Seven live" or "Six is unsettled". For the time being we'll leave proof of the two extremes to you and demonstrate the unsettled case.
If Black goes first, he can live. White is again rather stubborn to keep trying with 2 and 4, but after 5 Black has completely constructed the two eyes he needs to live.
White can start her attack with a hane at 1.
(Reducing your opponent's eye space like this should always be the very first option you consider when examining the status of one of his groups, by the way. See also the article on approaching a life and death problem the right way.).
Even if Black were to resist, White can keep his group dead.
unkx80: See also Two Space Notchers Not Fully Connected.
Marked spots: possible weaknesses
Marked stones: legs
aka the flying saucer shape. It is dead
Black can live
Black can live
Black can live
Black can live
Black can live
dead
Black can live
(...)
Black can live
Black is alive
(...)
However, 223WSNotcher shows a killing sequence for White.
Black is alive
(...)
See notchers for conventions and notcher code for the code.
211WW D - 212WW D - 213WW D -- 214WW ? -- 215WW ?
211WN D - 212WN D - 213WN LD - 214WN ? -- 215WN ??
211WS D - 212WS D - 213WS LD - 214WS ?? - 215WS ??
211NN D - 212NW D - 213NW LD - 214NW ? -- 215NW ??
211NS D - 212NN D - 213NN LD - 214NN ?? - 215NN ?
211SS D - 212NS D - 213NS LD - 214NS ?? - 215NS ?
xxxxx --- 212SW D - 213SW LD - 214SW ?? - 215SW ??
xxxxx --- 212SN D - 213SN LD - 214SN ?? - 215SN ?
xxxxx --- 212SS D - 213SS LD - 214SS ?? - 215SS ?
222WW D -- 223WW LD
222WN LD - 223WN L
222WS LD - 223WS L
222NN LD - 223NW L
222NS LD - 223NN L
222SS LD - 223NS L
xxxxx --- 223SW L
xxxxx --- 223SN L
xxxxx --- 223SS L